1. Field of the Invention
This invention relates to a process of producing a metal base laminate plate which is used as a printed circuit board for a keyboard device or a like device.
2. Description of the Prior Art
A keyboard device which is used as an input device for a personal computer, a word processor or the like normally incorporates therein a printed circuit board having a large number of through-holes perforated therein in which terminals of various key switches are fitted. In recent years, so-called metal base laminate plates which include a base of a metal plate coated with an insulator film are used by a progressively increasing quantity for such printed circuit boards. Since a metal base laminate plate includes a metal plate of iron, aluminum or some other suitable metal as a base, it is advantageous in that it can be bent readily and it has a high heat radiating effect and a high magnetic shielding effect. Accordingly, it can be estimated that demands for metal base laminate plates will further increase in the future.
FIG. 7 shows, in partial cross section, a conventional metal base laminate plate. Referring to FIG. 7, a metal base laminate plate 1 is principally composed of a metal plate 2 made of a metal such as iron or aluminum and having a large number of perforations 2a formed at predetermined locations thereof, an insulator film 3 coated on the metal plate 2, and a copper foil pattern 4 formed on a surface of the insulator film 3. Each of the perforations 2a of the metal plate 2 is coated at an inner wall thereof with the insulator film 3 to thus form a through-hole 5. Solder resist 6 is printed on a surface of the copper foil pattern 4 except lands. It is to be noted that the insulator film 3 is formed by pressurizing and heating a prepreg which is formed from a paper sheet or a glass fiber sheet in which a resin such as a phenolic resin is impregnated.
A process of producing such a metal base laminate plate 1 as described above is disclosed, for example, in Japanese Patent Laid-Open No. 61-92849. According to the process, a metal plate 2 in which a large number of perforations 2a are formed at predetermined locations is prepared, and a pair of prepregs are placed on the opposite faces of the metal plate 2. Then, a sheet of copper foil is placed on a surface of one of the prepregs, and a layered body of them is pressurized and heated to shape the same into a unitary member. In this instance, the perforations 2a are filled up by resin material flowed out of the prepregs, and as the resin in the prepregs cures, the insulating film 3 is formed. Subsequently, the copper foil is subjected to etching or the like to form the copper foil pattern 4, and then the solder resist 6 is printed on the surface of the copper foil pattern 4 except lands. After then, the resin material filled in the perforations 2a of the metal plate 2 is removed by boring by means of a drill or a like tool to form the through-holes 5. Finally, bending is performed using a twin roll bending machine or the like to complete the metal base laminate plate 1 of a predetermined configuration.
With the conventional process of production described above, however, the working efficiency is low and it is difficult to improve the productivity because a press step for a period of 30 minutes or so under a reduced pressure is required in order to form the insulator film 3 by pressurizing and heating the prepregs. Further, since air admitted in between the prepreg and the copper foil cannot be removed completely by such a pressing step, air bubbles may remain by a significant amount in the metal base laminate plate 1. Consequently, there is the possibility that the copper foil pattern 4 may be exfoliated from the insulator film 3, and accordingly the yield is low.